The present invention relates to a focal plane shutter blade device of a camera, and more particularly, the present invention relates to front and rear blinds that are used during an exposure operation of a camera.
In a conventional camera system, light is shielded from an aperture by a front slit blade and a front covering blade, an example of which is disclosed in Japanese Laid-Open Patent Publication No. 8-313972. More specifically, the aperture is shielded from the light by the front slit blade and the additional front covering blade while a front blind covers the aperture. Since the front slit blade and the front covering blade occupy a large portion of available space around the aperture, the distance between the blades and the aperture is relatively small, and therefore the run-up distance is short. This close proximity between the blades and the aperture tends to corrupt the ability of the front blade to begin opening the aperture during an exposure operation. Likewise, the close proximity of the blades and the aperture also causes difficulties when there is a necessity to increase the velocity of the front and rear blinds.
In a conventional shutter blade device, the aperture begins opening before the velocity of the front blind sufficiently increases, and therefore it is necessary to increase the velocity of the front blind. In this way, by increasing the velocity of the front blind, the time from when the front blind begins opening the aperture until the aperture is fully opened, or the time required for the shutter blades to traverse across the aperture, is decreased. This increased velocity is therefore necessary to stabilize the accuracy of the shutter speed and to increase the synchronization speed. Prior art cameras have attempted to increase the velocity of the front blind by increasing the spring force to drive the front blind. However, when the spring force is increased, the size of the shutter also increases. In addition, the increased spring force tends to have an adverse effect on both the durability and reliability of the shutter. Furthermore, in order to increase the spring force, the energy required for a shutter charge must also be increased. In a camera that includes a rotating motor and a shutter that are charged by a driving force of a battery, this increased energy requirement causes battery consumption to increase, which increases the frequency of battery replacement.
In an attempt to increase the velocity, a special lightweight material, such as a carbon fiber composite material, including carbon fiber reinforced plastics ("CFRP"), could be used for the front and rear blinds to reduce the weight of the shutter blind. This reduced weight would help to increase the velocities of the front and rear blinds without having to increase the spring force. But such materials are expensive and would therefore increase manufacturing costs.
In a camera that uses an auto focusing device, the auto focusing device is positioned in the bottom of a mirror box of the camera body. During an exposure operation, the front and rear blinds move from the upper section of the camera body, where the viewfinder section is positioned, towards the auto focusing device. In this type of focal plane shutter, as illustrated in FIG. 7, shutter blades 102, 103, 104, and 105 that have retreated from aperture 101 are overlappingly accommodated between a viewfinder 106 and an upper edge 101a of an aperture 101. Consequently, a distance D.sub.h between upper edge 101a and viewfinder 106 must be increased in order to extend an approaching distance D.sub.f for shutter blade 102. This increased distance increases the height of the camera body, which has the adverse effect of increasing the size of the camera.